1. Field of the Invention
The present invention relates to a lithium secondary battery using dope and undope phenomena of lithium and, more particularly, to an improvement of a nonaqueous electrolyte composing the lithium secondary battery.
2. Description of Related Art
Recently, in consideration of environmental problems such as air pollution, electric cars and hybrid cars tend to become popular, and accordingly, batteries as electric sources of these cars have been required to exhibit high performance. Lithium secondary batteries including positive electrodes of which active materials are lithium transition metal composition oxides, negative electrodes of which active materials are carbon materials, and nonaqueous electrolytes have a characteristic of high energy densities, and accordingly, the lithium secondary batteries have been greatly expected as high-performance batteries. Lithium secondary batteries which use LiCoO2 as positive active materials have been already applied practically to portable electronic devices or the like. The lithium secondary batteries have been forecasted to be also used as power secondary batteries for use in electric cars and hybrid cars. Accordingly, the development of lithium secondary batteries using LiNiO2 and LiMn2O4 which are respectively inexpensive as compared with LiCoO2 has been expected.
Lithium secondary batteries using lithium transition metal composition oxides as positive active materials, and carbon materials as negative active materials are generally provided with protection circuits for ensuring the safety thereof. Namely, when the lithium secondary batteries are overcharged, the voltage increases and the temperature of the interior thereof rises. Accordingly, the electrolytes may decompose to cause generation of flammable gas. To ensure the safety of such batteries in consideration that organic solvents having low flash points are used as electrolytes thereof, the lithium secondary batteries have been designed so as to stop charging before overcharging.
On the other hand, the secondary batteries as power electric sources for motor vehicles or the like are large size, and a large amount of electric current flows therein. Accordingly, the secondary batteries are required to have higher safety. The above-described protection circuits are effective in small-sized batteries for use in portable electronic devices or the like, but do not serve to sufficiently ensure the safety of large-sized batteries as power electric sources of motor vehicles on the like.
Under the above circumstances, to enhance the safety of the large-sized batteries, various trials have been made by changing the materials composing the batteries. For example, it has been proposed to enhance the thermal stability by substituting elements such as Al for one part of Ni sites in LiNiO2 as positive active materials, or to make the electrolytes inflammable, using phosphoric ester, alkane fluoride, halogenated cyclic carbonate or the like, each exhibiting self-fire-extinguishing properties, as solvents. These trials or proposals, however, have not succeeded in ensuring sufficient safety.
On the other hand, thiophene, pyrrole (Publication of unexamined JP patent application No. Sho 62-160671), vinylene carbonate (Publication of unexamined JP patent application No. Hei 8-45545), sultone compounds (Publication of unexamined JP patent application No. Hei 11-339850), etc. have been proposed as additives for use in forming films on electrodes. By adding one of these additives to electrolytes, so-called cycle characteristic of the secondary batteries, that is the characteristic of restraining the lowering of the discharge capacity even after the repeated charging and discharging, is improved. However, these additives do not serve to ensure the safety of the secondary batteries.
The present inventors have analyzed the behavior of the secondary batteries during overcharging. As a result, it has become clear that when overchanging, the voltage of the secondary batteries increases and the temperature therewithin rises to cause oxidation and reduction of the electrolyte around the positive and negative electrodes, thereby generating flammable gas, and to cause emission of oxygen gas from LiCoO2, LiNiO2 or the like as the positive active materials, thereby causing thermorunaway of the secondary batteries, and consequently causing damage of casings thereof. Accordingly, they have earnestly studied to restrain these phenomena, thereby ensuring the safety of the secondary batteries. As a result, they have found that by protecting both the positive and negative electrodes, the reaction with the electrolyte can be restrained, and accordingly the generation of the flammable gas due to the decomposition of the electrolytes, and the emission of oxygen gas from the positive active materials can be restrained.
The present invention has been made based on their these findings.
It is an object of the present invention to provide lithium secondary batteries of very high safety, of which positive active materials are lithium transition metal composite oxides, and negative active materials are carbon materials, and which are capable of restraining both the generation of flammable gas due to the decomposition of the electrolytes, and the emission of oxygen gas from the positive active materials even during overcharging.
The lithium secondary battery in accordance with the present invention includes a positive electrode of which an active material is a lithium transition metal composite oxide, a negative electrode of which an active material is a carbon material, and a nonaqueous electrolyte which is prepared by dissolving a lithium salt in an organic solvent. The nonaqueous electrolyte contains at least one kind of conductive polymer-forming monomers which have alkyl groups, and are electrochemically polymerizable on the positive electrode within a battery operation voltage, and at least one kind of film-forming agents which electrochemically decompose within the battery operation voltage to form films on a surface of the negative electrode.
More specifically, the lithium secondary battery in accordance with the present invention is prepared by adding a specific conductive polymer-forming monomer and a specific film-forming agent to the nonaqueous electrolyte, and electrochemically polymerizing the monomer on the positive electrode while decomposing the film-forming agent on the negative electrode to form a film thereon, under normal operation states of the battery.
In the present description, xe2x80x9cwithin a battery operation voltagexe2x80x9d means xe2x80x9cwithin the range of voltage for normally charging and discharging batteriesxe2x80x9d and means, as is different from the case when the battery is overcharged, or the like, charging and discharging are performed within the range of voltage which is reversibly chargeable and dischargeable. In the case of the lithium secondary battery using LiCoO2, LiNiO2, LiMn2O4 or the like as the positive active material, and using the carbon material as the negative active material, for example, the charging end voltage is about 4.0 to 4.2 V, and discharging end voltage is about 3.0 V. Namely, charging and discharging are performed within this voltage range.
The structure of the conductive polymer formed on the positive electrode has not been cleared up, but alkyl groups are provided therein, thereby forming polymer which exhibits flexibility. When the battery is overcharged, and the temperature therewithin rises, the polymer melts. The molten polymer covers the positive electrode to restrain the reaction with the electrolyte, whereby both the generation of flammable gas due to the oxidation of the electrolyte, and the emission of oxygen gas from the positive active material can be restrained.
The film-forming agent decomposes on the negative electrode to form a firm film on the surface of the negative electrode. The reduction of the electrolyte on the negative electrode during overcharging is considered to be resulted from that when lithium ions are doped by the carbon material as the negative active material, the organic solvent as the electrolyte is also captured by the carbon material with the lithium ions to generate interaction of the organic solvent captured and the carbon material. Accordingly, since the film thus formed exhibits a high permeability against the lithium ions, but a low permeability against the organic solvent, the solvent is restrained from entering the carbon material, and consequently, the generation of flammable gas due to the reduction of the electrolyte during overcharging can be restrained.
Accordingly, with the lithium secondary battery in accordance with the present invention, the generation of flammable gas due to the oxidation and reduction of the electrolyte during overcharging is restrained, and the emission of oxygen gas from the positive active material is restrained, too. Thus, a lithium secondary battery of very high safety is obtained.
As is apparent from the later-described experimental results, the capacity of the lithium secondary battery in accordance with the present invention does not lower greatly even after the repetition of charging and discharging cycles. Thus, the lithium secondary battery in accordance with the present invention exhibits an excellent cycle characteristic, especially at elevated temperatures. This is considered to be resulted from that by adding specific conductive polymer-forming monomer and film-forming agent to the nonaqueous electrolyte, cutting of conduction paths due to the repetition of charging and discharging cycles is restrained on the positive electrode while the self-discharging rate of the charged lithium ions is decreased on the negative electrode.